(1) Field of the Invention
The present invention relates to a technique for manufacturing optical modules provided with an optical device (such as a semiconductor laser and a photodiode) that are used in optical fiber communications, at low cost.
(2) Description of the Related Art
In manufacturing optical modules, one of active alignment and passive alignment is employed as a method in order to optically couple one end of an optical fiber and an optical device.
With the active alignment method, an optical fiber is coupled with an optical device at a position where the highest optical coupling efficiency is obtained. This position is determined in the following manner; one end of the optical fiber is moved three dimensionally for fine adjustment while light emitted from a light emitting unit enters the end of the optical fiber, and an output intensity of light that comes out from the other end is measured.
Aligning optical axes of the optical fiber and optical device using the active alignment method is suitable to manufacture optical modules used in long-distance and high-power transmission, because such optical modules have high optical coupling efficiency. However, there are problems with the active alignment method in terms of cost and mass productivity, because apparatuses for light-axis adjustment are expensive, and adjusting the light axis using the active alignment method takes time.
On the other hand, with the passive alignment method, the optical axes of the optical fiber and optical device are optically aligned, simply by disposing the optical fiber and optical device on a semiconductor substrate having a V-shaped groove.
Although not very high, the optical coupling efficiency between the optical fiber and optical device coupled using the passive alignment method is sufficient in cases in which a transmission distance is shorter than 20 km. Moreover, as an advantage in comparison with the active alignment method, the passive alignment method enables mass production of the optical modules at low cost.
FIG. 12 is a perspective view illustrating an example of a passive alignment optical module.
An optical module 100 includes a semiconductor laser 20, an optical fiber 30, a semiconductor substrate 101, a resin 103, and a wire 106.
The semiconductor substrate 101 includes a V-shaped groove 102, alignment markers 104, and electrodes 107 and 108.
The V-shaped groove 102 is formed using a mask alignment apparatus (not depicted) and an etching apparatus (not depicted). A width of the V-shaped groove 102 is determined by the mask alignment apparatus, and then the etching apparatus etches the V-shaped groove 102.
In the nature of crystalline structure of the semiconductor substrate 101, an angle of the etched groove is automatically determined according to the width of the groove. Therefore, it is possible to adjust the depth of the V-shaped groove 102 by depending on the width of the groove.
The alignment markers 104 allow a die-bonding apparatus (not shown in the drawing) to recognize an image of the position to place the semiconductor laser 20.
The semiconductor laser 20 is disposed junction-down on the electrode 108 of the semiconductor substrate 101 by the die-bonding apparatus.
“Junction-down” is a state in which a semiconductor laser (layered structure) is positioned with a main surface closer to a light-emitting layer (active layer) facing toward the semiconductor substrate. The main surface of the semiconductor laser closer to the light-emitting layer is hereinafter referred to as the front surface, and the other main surface is referred to as the back surface. Mounting the semiconductor laser with the front surface joined to the semiconductor substrate is called “junction-down mounting”. In contrast, “junction-up” is a state in which the front surface of the semiconductor laser faces away from the semiconductor substrate, and mounting the semiconductor laser with the back surface joined to the semiconductor substrate is called “junction-up mounting”.
The back surface of the semiconductor laser is usually polished. Due to the polishing, a distance from the back surface to the light-emitting layer varies for individual semiconductor devices.
This means the junction-up mounting requires an adjustment of the light-axes. Therefore, normally junction-down mounting is employed in the passive alignment method.
In general, a power electrode that receives driving current is formed on the front surface, and a ground electrode is formed on the back surface of the semiconductor laser 20.
A soldering paste is applied to the electrode 108 before the semiconductor laser 20 is disposed. In a reflow step after the disposition of the semiconductor laser 20, the power electrode of the semiconductor laser 20 and the electrode 108 are soldered.
Further, the ground electrode of the semiconductor laser 20 and the electrode 107 are wire-bonded.
The optical fiber 30 is disposed in the V-shaped groove 102, and then bonded to the semiconductor substrate 101 by the resin 103.
Moreover, in order to obtain as high an optical coupling efficiency as that obtained by the active alignment method using a simple structure, Japanese Laid-Open Patent Application No. H10-311936 discloses an optical module characterized in that an optical fiber and an optical device are optically coupled after being disposed in a guiding groove or a square groove that are formed with a high degree of accuracy.
For both of the above-described optical module 100 and the optical module disclosed in the patent document, semiconductor substrates made of Si and having excellent flatness and high precision workability are utilized.
While the optical module 100 manufactured by the passive alignment method as described above and the optical module disclosed in the patent document can be mass produced at low cost in comparison with the active alignment optical modules, the optical module 100 and the optical module disclosed in the patent document have a disadvantage of poor high-frequency properties.
The poor high-frequency properties can cause the following problem. In an optical module provided with a semiconductor laser, a high-frequency current is fed through a wiring pattern to drive the semiconductor laser at high speed. Moreover, the semiconductor laser cannot be driven if the high-frequency current flows to a parasitic capacitance formed between electrode patterns that sandwich an insulation layer on the semiconductor substrate.
Another problem is that the production cost of the semiconductor substrate (including costs for material, forming the V-shaped groove, flattening, and processing) is relatively expensive compared to substrates made of other material such as ceramic and metal.